1. Field of the invention
The present invention relates to a method of transmitting a frequency-modulated asynchronous NRZ signal, associating a transmitting frequency called center frequency to a given logic value and a frequency deviation which is sometimes positive and sometimes negative relative to the center frequency to the reverse logic value.
The invention further relates to a transmitter device for transmitting frequency-modulated asynchronous NRZ data and a receiver device for such data.
"Asynchronous" is understood to mean the fact that the signal may change from one logic state to another at any moment, irrespective of any clock.
2. Description of Related Art
There is a difficulty with a DC component, when one wishes to transmit a frequency-modulated asynchronous NRZ signal, due to the fact that the signal comprises an unequal number of bits in either one of the logic states. Let us suppose, for example, that a high-frequency oscillator is used, directly modulated by a two-level signal for which the frequency is more often deviated than centered: under the influence of its feedback loop, the oscillator drifts so that the deviated frequency approaches the nominal frequency. A partial solution is provided by a signal which is sometimes positive and sometimes negative for a given logic value. Such a signal is known to the expert: for example, the publication "Techniques de l'ingenieurs, Electronique" chapter E7100, page 4, shows a signal called bipolar signal whose "one" bits are alternately positive and negative. The problem is, however, not completely solved by utilizing such a signal, because an unbalance may continue to exist and, furthermore, with an asynchronous signal, the absence of a clock linked with the signal does not permit of counting its bits for generating alternately positive and negative bits. This problem could be solved, for example, by working at a low frequency with an oscillator without feedback, then transposing the produced frequency to higher values which would necessitate a relatively complex device.